1. Field of the Invention
The present invention relates in general to relays and in particular to a relay having a floating contactor.
2. Description of Related Art
FIG. 1 is a block diagram of a portion of a typical prior art integrated circuit (IC) tester 10 including a set of channels 12, one for each of several terminals of an IC device under test (DUT) 14. Each channel 12 includes a channel control and data acquisition circuit 16, a comparator 18 and a tristate driver 20. A relay 24 links an input of comparator 18 and an output of driver 20 to a DUT terminal 26. Another relay 25 connects a parametric measurement unit (PMU) 28 within channel 12 to DUT terminal 26. A host computer 30 communicates with the channel circuits 16 of each channel 12 via a parallel bus 32.
Tester 10 can carry out both digital logic and parametric tests on DUT 14. Before starting a digital logic test, the control and data acquisition circuit 16 of each channel 12 closes relay 24 and opens relay 25 to connect comparator 18 and driver 20 to DUT terminal 26 and to disconnect PMU 28 from terminal 26. Thereafter, during the digital logic test, the channel control signal may turn on driver 20 and signal it to send a logic test pattern to DUT terminal 26 when the DUT terminal 26 is acting as a DUT input. When terminal 26 is a DUT output, circuit 16 turns off driver 20 and supplies an xe2x80x9cexpectxe2x80x9d bit sequence to an input of comparator 18. Comparator 18 produces an output FAIL signal indicating whether successive states of the DUT output signal matches successive bits of the expect bit sequence. Circuit 16 either stores the FAIL data acquired during the test for later access by host computer 30 or immediately notifies host computer 30 when comparator 18 asserts the FAIL signal.
PMU 28 includes circuits for measuring analog characteristics of the DUT 14 at terminal 26 such as, for example, the DUT""s quiescent current. Before starting a parametric test, the channel control circuit 16 opens relay 24 and closes relay 25 to connect the channel""s PMU 28 to DUT terminal 26 and to disconnect comparator 18 and driver 20 from terminal 26. Host computer 30 then programs PMU 28 to carry out the parametric test and obtains test results from the PMU.
Relays 24 and 25 are normally preferred over solid state switches for routing signals between DUT 14, PMU 28, driver 20 and comparator 18 because a relay, having very low loss, does not substantially influence test results. We would like to position comparator 18, driver 20, relays 24 and 25, and circuit 16 as close as possible to DUT terminal 26 to minimize the signal path lengths between terminal 26, comparator 18 and driver 20. When the signal paths are too long, the signal delays they cause can make it difficult or impossible to provide the signal timing needed to properly test DUT 14, particularly when the DUT operates at a high speed. Thus to minimize signal path distances we want to use relays 24 and 25 that are as short as possible and which can be reached via short signal paths.
In some prior art testers, one or more channels 12 are implemented on each of a set of printed circuit boards (xe2x80x9cpin cardsxe2x80x9d) that are mounted in a cylindrical chassis to form a test head. FIG. 2 illustrates a simplified plan view of a typical test head 34. FIG. 3 is a partial sectional elevation view of the test head 34 of FIG. 2. FIGS. 4 and 5 are expanded front and side elevation views of a lower portion of one of a set of pin cards 36 mounted within test head 34. Pin cards 36 are radially distributed about a central axis 38 of test head 34 and positioned above an integrated circuit device under test (DUT) 14 mounted on a printed circuit board, xe2x80x9cload boardxe2x80x9d 39. A set of pogo pins 41 provide signal paths between relays 24 and 25 mounted on pin cards 36 and contact points on the surface of load board 39. Microstrip traces on load board 39 connect the contact points to terminals of DUT 14.
Relays 24, 25 are mounted near the lower edges of each pin card 36 as close as possible to central axis 38 to minimize the signal path distance to DUT 14. However from FIG. 2 we can see that the space between pin cards 36 is relatively limited near axis 38. Thus in order to position relays 24, 25 close to axis 38 we want to use relays that are relatively thin but which are fast and reliable.
FIG. 6 is a simplified sectional elevation view of a conventional reed relay 40 including a glass tube 42 containing a pair of conductive reeds 44 and 45 serving as the relay""s contacts 47. A wire 46 wraps many turns around tube 42 to form a coil 48. Reeds 44, 45 are normally spaced apart, but when a voltage is applied across opposite leads 50, 52 of coil 48, magnetic flux produced by the coil flexes reeds 44, 45 causing them to contact one another so that a current may flow through the relay contacts 47. A conductive sheath 43 partially surrounds tube 42 to provide a ground surface. The spacing between reeds 44, 45 and shield 43 influences the characteristic impedance of the transmission line formed by reeds 44 and 45 when they are in contact.
The magnetic force produced by coil 48 on reeds 44,45 is proportional to the product of the magnitude of the current passing through coil 48 and the number of turns of coil about tube 42. A large number of coil turns is provided to minimize the amount of current needed to operate relay 40. However the large number of turns contributes to the thickness of relays; a relay""s coil typically contributes more than half the thickness of the relay.
FIG. 7 is a schematic diagram of a typical circuit for driving coils of a set of N relays 40. One end of each relay""s coil 48 is connected to a voltage source 54 while the other end of the relay""s coil is connected to ground through one of a set of N switches 49 controlled by one of control signals C1-CN. For example when a control signal Cl turns on one of switches 49, the current passes through relay coil 48 thereby causing the relay""s contacts 47 to close. When control signal C1 turns off switch 49, current stops passing though coil 48 and allows contacts 47 to open.
When switch 49 opens, the magnetic field produced by coil 48 collapses producing a transient voltage spike across coil 48 that is limited by a diode 56 connected across the coil. Without diode 56 the voltage spike would pass though voltage source 54 and appear as undesirable noise in other circuits receiving power from voltage source 54. Reeds 44 and 45 are also subject to contact bounce, wear, sticking and stress failure.
The opposing faces of reeds 44 and 45 have capacitance when relay 48 is open and that xe2x80x9cstubxe2x80x9d capacitance can influence high frequency signals. Referring to FIG. 1, for example, when the relay 24 linking unit 16 is closed and the other relay 24 is open during high frequency tests, the stub capacitance of the open relay can distort signals passing between the DUT and driver and receiver 20 and 18.
Since reeds 44 and 45 large enough to carry large currents have substantial inertia, and since reed inertia slows relay operation, relay reed size represents a trade-off between relay speed and current carrying capacity. Reeds 44 and 45, tube 42, shield 43, coils 46 and diode 56 all contribute to the size of relay 40 and the bulk of that relay makes it difficult to concentrate several such relays into a small volume. Since relay bulk can limit the number relays 24 (FIG. 1) that can be placed in a small area near a DUT terminal, only a such few relays can be used in each channel 12. The limitation of number of relays 24 in turn limits the number of test components such as devices 16 and 28 that can alternatively access
What is needed is a compact, low-noise, low-stub capacitance, long-life relay for use as relays 24 and 25 of the integrated circuit tester of FIG. 1 and other applications which can switch relatively quickly for the amount of current it must carry and with little contact bounce.
A relay in accordance with the invention includes one or more conductive coils embedded in an insulating substrate having multiple horizontally disposed layers. The relay also includes at least one set of contacts bordering a space containing a contactor in which at least a portion of its surface is conductive and shaped to mate with the contacts. The contactor is xe2x80x9cfree-floatingxe2x80x9d (i.e., unattached to any other object) and free to move within the space adjacent to the contacts. The contactor includes material such as iron or nickel so that a magnetic field can apply a motive force on the contactor. Current passing through the coil or coils produces magnetic fields which can selectively either position the contactor within the space so that its conductive surface mates with the contacts to provide a signal path therebetween, or so that its conductive surface does not mate with the contacts and does not provide a signal path therebetween.
A relay in accordance with a first embodiment of the invention includes first and second coils. When a current passes through the first coil it produces a first magnetic field pulling the contactor onto the contacts. When current alternatively passes through the second coil it produces a second magnetic field pulling the contactor away from the contacts. Thus the switching state of the relay is determined by whether current passes through the first or second coil.
A relay in accordance with a second embodiment of the invention employs a spherical contactor having first and second hemispheres of opposite magnetic polarity. The first hemisphere has a conductive surface while the second hemisphere has a non-conductive surface. When current passes through the coil in a first direction it creates a first magnetic field forcing the conductive surface of the contactor""s first hemisphere onto the contacts thereby creating a signal path between the contacts. When current passes through the coil in a second direction it creates a second magnetic field forcing the non-conductive surface of the contactor""s second hemisphere onto the contacts thereby breaking the signal path between the contacts.
A multiple pole relay in accordance with at third embodiment of the invention includes a spherical contactor free to roll around a torroidal channel formed in the substrate. Several contacts are distributed around an output periphery of the channel while a common contact covers an inner surface of the channel. A separate coil is embedded in the substrate proximate to each contact. Whenever a current is applied to one of the coils, it creates a magnetic field attracting the contactor so that the contactor positions itself to provide a conductive path between the contact proximate to that coil and the central contact.
When a relay in accordance with the invention employs a very small contactor which can be moved by relatively small magnetic fields, the relay""s coils and cores can be relatively small. Thus many such relays can be concentrated into a relatively small volume. Since the relay""s coils, cores, and contacts, and in some embodiments the contactor, are embedded in a substrate such as a printed circuit board, the relay requires little or no space on the surface of the substrate. Since it does not include any springs, reeds or other parts that substantially deform wherein making when breaking a signal path, a relay in accordance with the invention is less subject to contact bounce and material stress failures than conventional relays.
It is accordingly an object of the invention to provide a very compact, high-speed, low stub capacitance, long-lived relay that is relatively unaffected by contact bounce.
The claims portion of this specification particularly points out and distinctly claims the subject matter of the present invention. However those skilled in the art will best understand both the organization and method of operation of the invention, together with further advantages and objects thereof, by reading the remaining portions of the specification in view of the accompanying drawing(s) wherein like reference characters refer to like elements.